Fatigue failure in high pressure equipment is a common problem which becomes especially notorious when frequent pressure fluctuations are encountered. One exemplary commercial field where extensive cyclic pressure fluctuations are prevalent is the high pressure polymerization of ethylene.
The most common form of equipment used for high pressure ethylene polymerization is the tubular reactor which consists of long sections of heavy walled tubing interconnected in series. In order to permit intermediate entry and lateral access to the reaction zone at points along its length without damage to said heavy wall tubing, it has been conventional practice to provide between the flanged ends of adjacent sections of tubing rather massive connection blocks, such as block 38, illustrated in FIG. 2 of U.S. Pat. No. 4,008,049. Because of the stress concentration effects arising from the thermocouple access ports or the like drilled through the side wall thereof, these prior art connection blocks had to be constructed with exceptionally thick walls in order to prevent fatigue cracks from developing therein during service.